The four Sound Ninja used a barrier ninjutsu called the Four Violet Flames Formation. Once the purple barrier sealed completely, ordinary jōnin could not break through it, but Madara Uchiha certainly did not fall into that category.
Yet once the Third Hokage threw back the kunai Madara had given him, Madara no longer needed to rescue him. Doing so would only dishonor the Third Hokage's spirit of sacrifice.
The kunai bore Madara Uchiha's Flying Thunder God Formula. Whenever the Third Hokage needed help, Madara could easily reach his side through it. By throwing it back of his own accord, the Third Hokage was declaring that he had made his peace with death and intended to stake his life on this battle.
"Orochimaru, it's been a long time."
Within the Four Violet Flames Formation, the Third Hokage calmly looked at Orochimaru, who was holding him hostage. Although Orochimaru was still disguised as the Fourth Kazekage, the Third Hokage exposed his identity outright.
"Things have already come this far. There's no need to keep pretending to be the Fourth Kazekage, is there?"
Several days earlier, Madara Uchiha had obtained secret intelligence about Otogakure and Sunagakure gathering shinobi on a massive scale. At the same time, his subordinates had found the Fourth Kazekage's corpse, and Madara had learned of the Konoha Crush that Orochimaru and Sunagakure had jointly planned.
Madara had relayed every detail to the Third Hokage. In the end, the village leadership decided to turn their enemy's scheme against them. All of Konoha had prepared for this day, with the aim of seeing just what Orochimaru would attempt.
The plan had called for Madara and the Third Hokage to face Orochimaru together, but clearly, the Third Hokage had made plans of his own for today.
Orochimaru was the Third Hokage's student.